Hi.
I used Gliptone on my tan leather - it had great reviews but struggled to get the worst of it off..
I simply used Flash cleaner first ( although it isn't suitable it did a good job ) and used the Gliptone products to maintain the leather after...

Hi yes I think the leather is costed with something (as well as dirt in my case)
I have just bought the car and whilst the leather generally is not too bad it has a dirty drivers seat.
Also looking for tips on removing the smoke smell from previous owners and cleaning headliner if possible

As for smoke, you can clean the headlining in situ (don't try taking it out, you'll soon regret it) but the smoke stink will be in everything, seats, carpets, sound deadening/insulation, door cards, dash, heater matrix.... So if you're not planning on stripping the entire interior of your car there's not much you can do about it (besides trying to mask it with something even worse) other than accept the fact your car will stink but it will fade eventually.

Most leather seats in cars are coated with a sealant so any "conditioners" will just sit on the surface and transfer to your clothing. A simple test is to allow a drop of water onto the leather surface; if it soaks straight in then that's uncoated old school leather but if it just sits on the surface then you have coated leather and conditioners will not soak in as a treatment.

For cleaning I'd recommend a leather cleaning brush, it looks like a nail brush but has finer bristles, use the brush with a leather 'cleaner' fluid to agitate the surface and get into the grain pattern, keep wiping the excess with a microfibre towel etc.

For a final treatment you can use a water based all purpose dressing just to give an 'as new' finish, I've began using Auto Finesse products this year with very good results.

Normally I use the Leatherique products, the rejuvenator is to make the leather more softy and to extract all dirty from the leather. After you use prestine clean, is a cleaner and conditioner normally used after the application of the rejuvenator.